Thanks for the updated information. Since you have grown this plant the same way for many years, and been successful doing so, I would rule out growing conditions as being a cause. I don't quite understand the watering schedule though. I water all my ficus plants when the soil appears dry, not before. I usually have to water my 25+ old "mother" fiddle leaf ficus every week year 'round. But as I said, watering probably is not the issue here.
Here are two guesses as to your problem.
1. You could have a buildup of fertilizer minerals in the soil and that buildup is literally poisoning your plant. I have never used those fertilizer stakes for indoor plants and thus don't know about the constant fertilizing they impart to the soil.
2. Every plant has a life-span and it may be that yours has simply reached that plateau. Your plant is over 20 years old, and for some ficus (such as the Benjamina), this is approaching their life-span. Since you saved it from certain death on that curb (and don't know how old it was then), your plant could be much older than 20 years.
Do you trim your plant back every year? Ficus are stimulated to produce more growth and healthier growth when they are trimmed.
If your problem is mineral toxicity, you should remove all those spikes and flush the soil with clear water when the soil looks dry. I would aggressively trim back all the branches (but not the main "trunk". Alternatively, replace the soil completely, trimming the roots and branches extensively, and repot with fresh potting soil (one without fertilizer).
Good luck, Julie.
